Density

Underwater Fireworks

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Fireworks are fabulous! We love watching the colors explode and gently float down through the air, but we can’t watch fireworks all the time. Underwater fireworks are something we can watch any time, even in our kitchen!

The Experiment

Supplies: Water, vegetable oil, food coloring (any color), a large clear glass, a second smaller glass, a fork

What to do: Fill the large glass almost to the top with room-temperature water. Pour 2 tablespoons of oil into the other glass. Add 2 drops of food coloring to the glass with the oil. Vigorously stir the oil into the food coloring using a fork. Stop once you break the food coloring into little drops. Pour the oil and coloring mixture into the tall glass. Now watch! The food coloring will slowly sink in the glass, with each droplet expanding outwards as it falls.

What is happening: Food coloring dissolves in water, but not in oil. This has to do with the incompatibility of the molecular structures of the water and the oil. When you pour in the food coloring/oil mixture, the oil will float at the top of the water because it is less dense. The food coloring will begin to dissolve once it sinks through the oil and into the water.

Take It Further

Variations on this demonstration:

  • Try mixing in one drop each of two different food colorings.
  • What happens if you omit the oil and drop the food coloring directly into the water?
  • Try varying the size or shape of the water glass, the amount of the oil, or the amount of the food coloring. Just remember, when you are manipulating variables to only change one thing at a time!

Links

For a more detailed explanation of the miscibility of fluids, along with a more expansive version of this demonstration, head over to the Scientific American website.

Deep Diving Raisins

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In the Exploring Density experiment, we looked at how different liquids have different densities, and how the different densities helped the liquids stay in separate layers, even when combined in the same container. In this experiment, we will explore how the density of an object can be altered without changing the object’s mass.

Before we begin, let’s review some key terms. Volume is how much space an object takes up. Mass is how much an object weighs. Density is the comparison of mass to volume. Something that doesn’t weigh that much but takes up a lot of space has low density. Something that doesn’t take up a lot space but weighs a lot has high density. In the previous comparison, we looked at a 1-pound bag of marshmallows vs. a baseball. Both have the same mass (or weight), but the baseball contains that mass in a much smaller volume (or space) than the marshmallows, so the baseball has a higher density.

If you can increase the volume of an object without increasing its mass, you will change its density. Imagine a balloon. If you weigh a balloon before you blow it up and weigh it again after you blow it up, you will see that the weight of the balloon increased only a little bit (due to the weight of the air inside the balloon), but the volume of the balloon increased dramatically. The uninflated balloon has a much greater density than the inflated balloon because the weight is confined to a much smaller space. Let’s try increasing the volume of something without increasing the mass at all!

The Experiment

Supplies: Two clear drinking glasses, eight raisins, tap water, a clear soda drink (club soda, 7-Up, etc.)

What to do: Fill one glass with water. Fill the other glass with the soda drink. Drop four raisins in each glass. Observe the raisins. What did the raisins in each glass do? How long did it take for the raisins to stop?

What is happening: When you first drop the raisins into each glass, they sink to the bottom of the glass because they have a greater density than the liquids they are in. The raisins in the glass of water do nothing, but the raisins in the soda begin to move after a short time. Raisins have a rough, dented surface. If you inspect the raisins in the water glass, you should see some air bubbles attached to each raisin. There are not enough bubbles on the raisins in the water glass to affect the raisins’ density, and there is no other source of bubbles in the water. Soda is carbonated, meaning it has carbon dioxide gas as one of its ingredients. Carbon dioxide gas is what makes soda “bubbly”. The soda releases carbon dioxide bubbles, and these bubbles attach to the rough surface of the raisins. The carbon dioxide bubbles increase the volume of each raisin, but the mass of each raisin stays relatively the same. When the volume increases but the mass does not, the overall density of the raisin is lowered. The raisins are now less dense than the soda, so they rise to the surface.

Once the raisins get to the surface of the soda, the carbon dioxide bubbles pop, causing the raisins’ density to change again, and they sink as a result. Once the raisins reach the bottom of the glass again, the process repeats itself, sending the raisins back toward the surface of the soda. The raisins will bob up and down for several minutes, until all of the carbon dioxide has escaped and the soda is flat. This experiment demonstrates how an increase in volume can lead to a decrease in density in an object, as long as the mass of that object is not significantly affected.

Take It Further

Try putting the raisins in a jar with a lid or directly into a bottle of soda. What happens to the raisins when you put the lid or cap back on? What happens when you take it back off? When put the raisins into a container and seal the container, the raisins will eventually stop the rising and sinking cycle. When it is in a glass, the carbon dioxide released by the soda can escape into the atmosphere. In a closed container, some carbon dioxide gets released, but it cannot leave the bottle, so pressure builds up in the space between the surface of the soda and the bottle cap. When you open the bottle, the hissing noise you hear is the built-up carbon dioxide escaping the enclosed space. As long as the cap is on the soda bottle, the contents of the bottle are under pressure. That pressure prevents carbon dioxide bubbles from forming, and any bubbles that do form cannot grow as large as they would in an open container or glass. Once the container is opened and the pressure released, then the bubbles are free to form again, and the raisins will resume their floating and sinking cycle!

Try this experiment with other dried fruits, like cranberries, or other small fruits or nuts, like grapes, blueberries, almonds, or peanuts. Are the results the same?

Links

To learn more about density, head to Kiddle Encyclopedia!

Exploring Density

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Density describes the relationship between a substance’s mass, or weight, and its volume, or how much space it takes up. Things that are more dense take up less space than things with less density. To visualize density, let’s compare baseballs and marshmallows. A regular baseball weighs about one pound. It is small and compact and fits in the palm of your hand. A 16 oz. bag of marshmallows also weighs one pound, but because there is more air incorporated into the marshmallows, they are less dense. One pound of marshmallows takes up a lot more space, or volume, than a one-pound baseball.

When you combine substances that have different densities, the substances with the greatest density tend to move toward the bottom, while those with lesser densities tend to rise to the top. Gases tend to be lighter and less dense than liquids. Liquids tend to be lighter and less dense than solids. Even within these groups, there are a variety of densities.

The Experiments

Simple Experiment

Supplies: A clear jar with a lid, vegetable oil, water, food coloring (optional).

What to do: Fill the jar about half-full with water. Add food coloring, if desired. Pour in vegetable oil until the jar is almost full. Put the lid on the jar and MAKE SURE IT IS TIGHT. Give the jar a good shake so that the water and oil are thoroughly mixed. Set the jar where it won’t be disturbed and observe the liquid.

What is happening: The oil is less dense than the water, so it rises up to “float” on the surface of the water. The water and oil do not mix because of the molecular properties of each compound. Water molecules tend to want to “stick” to other water molecules, while oil molecules tend to want to stick to other oil molecules. This is because of something called molecular polarity, where the structures of the two molecules are not compatible. They push each other away, similar to a pair of magnets that won’t stick together. The longer the jar sits, the more the water and oil will sort themselves out, until they are completely separate again.

Complex Experiment

Supplies: A large jar or clear glass cylinder, liquid measuring cup with pour spout, a turkey baster, different colors of food coloring, honey or molasses, light corn syrup (Kayro), blue liquid dish soap, rubbing alcohol, yellow corn oil, water.

What to do: Pour 1 cup of honey or molasses into the bottom of your jar. Measure out 1 cup of light corn syrup and add some red food coloring. Stir until well combined. Carefully pour the corn syrup into the jar, making sure to avoid hitting the sides of the jar. Measure out 1 cup of dish soap. Slowly add the dish soap, again avoiding the sides of the jar. Measure out 1 cup of water and add to the jar, but this time use the turkey baster to slowly drizzle the water down the side of the jar. Measure out 1 cup of corn oil and add to the jar, again using the turkey baster. Finally, measure out 1 cup of rubbing alcohol. Add green food coloring and stir well. Add the rubbing alcohol into the jar using the turkey baster.

What is happening: Different liquids have different densities. Liquids like honey and dish soap are more dense than water, while other liquids, like rubbing alcohol and vegetable oil are less dense and will float above the water. Adding the food coloring helps distinguish the different layers. The various densities of the different liquids is what keeps the layers separate.

Links

You could take the Density Column experiment one step further by dropping in solid objects to see where they land, OR you could just watch this great video from the Bearded Science Guy.